about 8 shots...9-10 centaurs come...let's say we have 4 heroes and attack with a Knight or Barbarian...40 archers (centaurs or centaurs and archers/orcs) with 2 damage each = 80 damage, that's 2 elementals per turn...multiply by 8 shots...16 elementals with arrows...the rest (a few) in melee with gargoyles and centaurs.
1. A hero gets 6-10 centaurs (on average 8), so from 4 heroes, there are only 32!
2. Centaurs deal 1-2 damage, on average 1.5! But a centaur has 3 attacks, and an elemental has about 8 defense! So, the damage will be barely 1 elemental - or even less, somewhere around 35 damage.
So, even if there are only 20 elementals, and we have another 10-12 gargoyles with the centaurs, then at best, we can kill about 10 elementals! :)